The invention pertains to a device for measuring the angle of rotation or a value derived therefrom in an electrical machine equipped with a commutator. This machine can be a motor or a dynamo or generator. Essential for the invention is that a commutator is provided that is equipped in a known manner with electrically conductive contacts, via which currents are conducted in temporal sequence that produce a more or less constantly alternating field. Conversely, in the case of a dynamo, the commutator can receive the generated current. The invention can also be used with annular sliding contacts.
It is often desirable to determine the rotary status of the rotor or an angular value for the rotor of an electrical machine derived therefrom without regard as to whether the machine is operated in generator or motor mode. To this end, it is known from DE-OS 41 03 561 that the shaft of a motor can be connected to magnets, with Hall elements provided in the stator associated with these magnets. In DE-OS 35 39 390, magnets are mounted on the shaft of a tachogenerator, the rotary status of which is scanned by an inductive sensor, while a commutator is axially offset on the shaft (see FIG. 1.)
Since the space available in small motors is often very limited, the objective is to integrate the rotary angle measurement device into other components of the electrical machine whenever possible.
Accordingly, the invention is based on a rotary angle measurement device. In realization of the objective, the invention proposes that the segmented basic body of the commutator be permanently magnetized, at least sectionally, and that the stator of the machine be equipped with a sensor responding to the rotary status of the commutator.
Therefore, the invention consists principally in utilizing the fact that the basic body of a commutator is fully capable of being magnetized without impairment of the primary function of the commutator, namely providing contact of its segments with the contact brushes.
It follows according to the invention that it is readily possible, in addition to the task of current conductivity, to integrate into the commutator the additional formation of a rotating magnetic field.
Inasmuch as a rotating magnetic field is already present in most electric machines, it is theoretically possible to make do without a magnetic field specially integrated into the commutator. However, the magnetic field specially integrated into the commutator is particularly capable of providing a field strength sufficient to activate a Hall element, whereby the Hall element could be located in the stator in the vicinity of the commutator. Of course, a better rotating field measurement can be achieved with several Hall elements. The invention is not limited to the following embodiments. With the measurement device according to the invention, the rotary speed, the rotary acceleration or any other value of the rotor derivable from its rotary status can also be determined.
The segments of the commutator themselves must not be electrically connected via the commutator. This can be realized by enclosing the individual segments in an insulating layer. However, an advantageous modification of the invention is recommended, according to which the basic body consists of an insulating material that is nevertheless so configured that it permits the magnetic flux to pass through the basic body. The basic body preferably consists of plastic. For the positioning of one or more magnets inside the basic body in the commutator in advantageous refinement, a series of measures are available. To this end, at least one prefabricated magnet be fitted into an appropriate recess in the commutator.
However, the basic body can also be molded from a magnetizable, electrically nonconductive material and then permanently magnetized or subjected to a magnetization during the molding process. It is alternatively proposed that the basic body consist of a sintered magnet subjected to a magnetization during or after the sintering process.
The special advantage of the invention lies in the fact that the sensor, especially a Hall sensor, can lie in the same plane as the carbon brushes. This facilitates shortening the length of the motor. The sensors can also be spatially separated farther from the interference suppressors located on the rear end shield of the motor. In this manner, the sensors are rendered less subject to the influence of the suppressors.